Pharaoh means great or royal house.
So what's in a name? Everything if you lived in ancient Egypt. Kings were much more than mortal men in ancient times. They were the divine ones. These ideologies stem back to the old legends of Osiris. As we all know, Osiris was the good guy. He was the great, wise, and all knowing king of Egypt. His very jealous brother Seth killed and dismembered him. It is written in some books that Isis along with the help of Anubis, reassembled him. Then Isis with the help of Nepthys breathed life back into Osiris long enough for him to impregnate Isis. And as the story goes she gives birth to Horus. Then Osiris is off to rule the underworld.
The kings ultimate function was to conform to the role of Horus, to become the living incarnation of Horus. It was understood that being of divine origin would guarantee Egypt's prosperity. Only the pharaoh could speak directly to the gods through divine right.